This invention relates in general to developing devices of electrophotographic printing apparatus, and more particularly the invention is concerned with a method and an apparatus for detecting the presence or absence of a developing agent within a developing agent container for dispensing the developing agent or for determining the residual amount thereof within such a container.
In a developing device of the dry developing agent type for an electrophotographic copying apparatus, for example, it is necessary to continuously supply a developing agent to a developing station to compensate for the consumption of the developing agent required for effecting copying, in order to keep the constant density of images formed by copying. To meet this requirement, it is essential that the developing agent be present in the developing agent container at all times. If the container becomes empty, then the absence of the developing agent should be detected and the container should be replenished with a fresh supply of developing agent at once. However, a difficulty is encountered in accurately determining and indicating the residual amount of the developing agent of the dry type within the container because such a developing agent tends to form into solid masses. In one method known in the art for determining the residual amount of the developing agent, the container is made of a transparent material so as to enable the operator to determine the residual amount by visual examination. However, since the operator is unable to keep the residual amount of the developing agent under surveillance at all times, there is no guarantee that the residual amount can be determined accurately by this method.
Proposals have hitherto been made to use various methods for detecting the presence or absence of a developing agent which vary depending on the types of the developing agents to be handled.
For example, one method consists in measuring the weight of the container in detecting the presence or absence of a developing agent. Another method relies on the detection of the liquid level in the container when the developing agent is in a liquid state, still another method uses a permanent magnet for determining a magnetic attracting force thereof for the developing agent when the agent contains a magnetic carrier, and a further method utilizes a change in the impedance between two electrodes, with one being mounted in the container and the other on the wall of the container. However, these methods of the prior art all have defects. Accurate determination of the residual amount of a developing agent is hampered by mechanical vibration in the case of the method consisting in measuring the weight of the container, by fluctuations in the liquid level in the case of the method relying on the detection of the liquid level, and by the lack of fluidity of the developing agent containing a magnetic carrier in the case of the method relying on the determination of a magnetic attracting force of a permanent magnet when the residual amount of the developing agent approaches a level at which a further supply of the developing agent to the container is necessitated.
The method utilizing the magnetic attracting force of a permanent magnet mounted in the vicinity of a toner container for the magnetic developing agent has disadvantages, in addition to the aforementioned disadvantage, in that the spilled developing agent adheres to the magnet and weakens its attracting force and causes a misoperation to occur and that the attracting force of the magnet interferes with the fluidity of the developing agent. In the case of a magnetic developing agent, the use of a magnetism sensing switch which is subjected to the magnetic flux from the magnet may be conceived for determining the residual amount of the developing agent. However, this method also has the aforementioned disadvantages associated with the use of the magnet.